Photonic Devices Monolithically Integrated with Cmos

ABSTRACT

Photonic devices monolithically integrated with CMOS are disclosed, including sub-100nm CMOS, with active layers comprising acceleration regions, light emission and absorption layers, and optional energy filtering regions. Light emission or absorption is controlled by an applied voltage to deposited films on a pre-defined CMOS active area of a substrate, such as bulk Si, bulk Ge, Thick-Film SOI, Thin-Film SOI, Thin-Film GOI.

BACKGROUND OF THE INVENTION

The present invention relates to light emission from semiconductorjunctions in general, and in particular when those junctions areoperated in the avalanche mode, as the active regions of Avalanche LightEmitting Diodes (ALEDs), thereby enabling light emission from indirectbandgap materials. It relates to the design of device layers and layout,as well as the method of fabrication, suitable for monolithicintegration of ALEDs with sub-micron and sub-100 nm CMOS technologies,forming “Light Emitting Elements” (hereafter referred to as LIXELs).LIXELs can be implemented with Silicon bulk substrates, Thick-Film SOIsubstrates, or ultra-thin-film Silicon-On-Insulator (SOI) substrates, aswell as with Germanium bulk substrates or ultra-thin-filmGermanium-On-Insulator (GOI) substrates. Thin-Film GOI substrates aregood candidates to be the used for sub-45 nm CMOS technology.

In the early years of semiconductor technology, it was noticed thatsilicon junctions operated in the avalanche mode emit white light. Infact it seems that light emission takes place across a large region ofthe electromagnetic spectrum, from the Long Wavelength Infra-Red (LWIR)to the Ultra-Violet (UV). Such wide interval of photon energies is anindication that different physical mechanisms, with differentprobabilities and efficiencies, are responsible for the emission ofphotons. Recent reviews of this topic can be found in: N. Akil, S. E.Kerns, D. V. Kerns, Jr., A. Hoffmann, J.-P. Charles, “A MultimechanismModel for Photon Generation by Silicon Junctions in AvalancheBreakdown”, IEEE Trans. on Elect. Dev., Vol. 46, No. 5, May 1999, pp.1022-1028, and M. de la Bardonnie, D. Jiang, S. E. Kerns, D. V. Kerns,Jr., P. Mialhe, J.-P. Charles, A. Hoffman, “On the Aging of AvalancheLight Emission from Silicon Junctions”, IEEE Trans. on Elect. Dev., Vol.46, No. 6, June 1999, pp. 1234-1239.

It is thought that some of those mechanisms are: (1) Interbandtransitions between (1a) hot electrons and thermal holes, (1b) hot holesand thermal electrons, (1c) hot electrons and hot holes; (2) Intrabandtransitions, (2a) in the conduction band, and/or (2b) in the valenceband; (3) Brehmstrahlung due to scattering of hot carriers by ionizedimpurities. Even though there has been ample experimental evidence sincethe 1950's that silicon can emit light, the efficiency has always beenvery low: roughly only 1 in 10⁷ recombinations across the bandgap emitlight. This low efficiency is tied to the details of the band structureof silicon, namely the smallest bandgap is indirect at 1.1 eV, to devicedesign/geometry, and to process architecture. Conventional avalanchelight emitting devices are made by ion implantation into a bulksubstrate, to make either lateral or vertical pn-junctions. In eithercase, the location which emits light can be hundreds of nanometers awayfrom the substrate surface, and consequently photons with energy largerthan the minimum bandgap of the substrate are absorbed, thereby severelyreducing the external power efficiency.

For all the reasons mentioned above, band-structure, device design, andprocess architecture, it has been impossible to take advantage forpractical applications, of light emission from silicon junctionsoperated in the avalanche mode. On the other hand, conventional CMOStechnology is not amenable to the integration of other semiconductormaterials for the purpose of bandgap engineering of pn-junctions. Forthis reason full monolithic integration of efficient light emittingdevices with CMOS has not been possible.

The present invention, based on the device and process architecturesdisclosed in WO 2002/33755, and in WO 2004/027879, and the new layoutdesigns disclosed in a co-pending application, presents a new method offabrication, device layers, and layout designs that enable themonolithic integration of ALEDs with advanced CMOS, including sub-100 nmtechnologies, in which the light emitting regions can be made ofmaterials other than the semiconductor substrate (e.g., silicon orgermanium). It also discloses optimized doping and heterojunctionprofiles for the purpose of increased efficiency of light emission, aswell as optimized profiles for the purpose of light emission in certainranges of wavelengths, namely in the 1.3 μm and 1.55 μm ranges.

The monolithic integration of ALEDs with advanced CMOS technology, inone exemplary implementation, requires only three additional masks, withrespect to the number of masks required for the CMOS technology inquestion. It has been experimentally verified that the avalanchephoto-diodes described in WO 2002/33755 and WO 2004/027879, with one ofthe layouts described in co-pending application, do emit light undercertain conditions of operation.

SUMMARY OF THE INVENTION

An object of the present invention is a new process architecture for thefabrication of photonic devices, that is compatible with sub-micron andsub-100 nm CMOS technologies and improves the intrinsic and extrinsicefficiencies of light emission by avalanching junctions.

-   -   1. Substrate can be Si bulk or SOI, Ge bulk or GOI, SiGe virtual        substrates—on bulk silicon or on insulator, etc. The type of        substrate to be used is related to the requirements of the CMOS        technology that the device is to be monolithically integrated        with.    -   2. For films deposited on bulk substrates, the acceleration        region (region of high electric filed) can be located inside the        substrate. In this case, the deposited film may include only the        impact ionization region, or it may also include a region for        “energy filtering”.    -   3. In the deposited film, the region designed to maximize impact        ionization, may have n-type or p-type conductivity.    -   4. For films deposited on thin-film or ultra-thin-film SOI or        GOI substrates, the doping and hetero-junction profiles of the        deposited films can be optimized for front-side light emission,        or for back-side light emission, or for both.    -   5. For front-side light emission from films deposited on        thin-film or ultra-thin-film SOI or GOI substrates, the films        incorporate the “acceleration region”, optionally an energy        filtering region, the region in which light emission is to take        place, which is at the same time an electrode.    -   6. For back-side light emission from films deposited on        thin-film or ultra-thin-film SO or GOI substrates, the films        incorporate the optional energy filtering region, the        acceleration region and the electrode opposite to the one in        which light emission takes place.    -   7. For front-side and back-side light emission from films        deposited on thin-film or ultra-thin-film SOI or GOI substrates,        the films incorporate the acceleration region, and the top        electrode. The top and bottom electrodes must be suitable for        light emission, and the acceleration region must be suitable for        light emission on both of its ends. Optional energy filtering        regions may be placed at both ends of the acceleration region,        or just at one end.    -   8. Regardless of substrate used, the acceleration region should        be mono-crystalline, because charge carrier mobility should be        as high as possible.    -   9. Regardless of substrate used, the region in which light        emission takes place should be as thin as possible to minimize        lateral light emission through the side walls.    -   10. The region in which light emission takes place can be        bandgap engineered so that the photon energy of the emitted        light can be below the threshold for interband absorption in the        substrate.

Another object of the present invention is a new device architecture,compatible with sub-micron and sub-100 nm CMOS technologies, thatimprove the intrinsic and extrinsic efficiencies of light emission byavalanching junctions.

-   -   1. The region in which light emission by avalanching takes place        is a thin-film, deposited on an active area.    -   2. Electric field and avalanche current, are perpendicular to        substrate surface;    -   3. The deposited film in which light emission by avalanching        takes place, can be a pure material, a random alloy, or a        short-period superlattice.    -   4. The deposition of the light emitting film can be engineered        to form pseudomorphic crystalline films, poly-crystalline films,        or amorphous films. It is also possible to have a combination of        films of different materials. For example, pseudomorphic        crystalline followed by poly-crystalline, or pseudomorphic        crystalline followed by amorphous, or pseudomorphic crystalline        followed by poly-crystalline, followed by amorphous. In        addition, it is also possible to deposited a crystalline film        and subsequently, with suitable processing, make it        nano-crystalline or porous.    -   5. During the deposition process, the heterojunction and doping        profiles can be optimized, for one or more of the physical        mechanisms involved in light emission, in order to maximize the        intrinsic efficiency of the light emission process(es).    -   6. The heterojunction and doping profiles of the deposited        film(s) can be optimized for efficient light emission in a        particular range of wavelengths.    -   7. The heterojunction and doping profiles of the deposited        film(s) can designed such that the deposited film is only used        as the location for impact ionization.    -   8. The heterojunction and doping profiles of the deposited film        (s), in addition to the location for impact ionization, can also        incorporate regions that act as “energy filters”, that is,        regions which allow charge carriers with only a certain energy        range, to move to the region in which impact ionization (i.e.        avalanching) takes place.    -   9. The heterojunction and doping profiles of the deposited film,        in addition to the location for impact ionization, can also        incorporate the regions in which the charge carriers are        accelerated.    -   10. The heterojunction and doping profiles of the deposited        film, in addition to the location for impact ionization, can        incorporate regions that act as “energy filters”, and also        incorporate the regions in which the charge carriers are        accelerated.    -   11. The same device, when biased above the breakdown voltage        emits light, and when biased below the breakdown voltage, can        operate as an avalanche photo-diode.

Yet another object of the present invention is a new layoutarchitecture, compatible with sub-micron and sub-100 nm CMOStechnologies, that improve the intrinsic and extrinsic efficiencies oflight emission by avalanching junctions.

-   -   1. For monolithic integration in a BiCMOS process with a        vertical Bipolar device, typical of SiGe (or SiGeC) BiCMOS        process technologies, the active area for the light emitter can        be either a CMOS active area or a Bipolar active area.    -   2. For monolithic integration in a pure CMOS process, the active        area is a standard CMOS active area.    -   3. For monolithic integration with a pure CMOS process on bulk        wafers, or thick-film SOI wafers, a conductive path to the        bottom electrode of the light-emitting device is implemented        with a well implant.        -   3a. For a p-type deposited film, the active area is n-type            on a p-substrate. A conductive path to the n-type active            area is implemented with a n-Well implant that overlaps both            sides of a portion of the shallow trench isolation            surrounding the n-type active area.        -   3b. For a n-type deposited film, the active area is p-type,            on a n-substrate, or n-Well on a triple-well on p-substrate            technology. A conductive path to the p-type active area is            implemented with a p-Well implant that overlaps both sides            of the shallow trench isolation surrounding the p-type            active area.        -   3c. With n-type active areas, the n-Well implant can also            overlap the source/drain region of a N-MOSFET on an adjacent            active area, thereby providing an extremely compact            arrangement of the light emitting device coupled to a            switching element.        -   3d. With p-type active areas, the p-Well implant can also            overlap the source/drain region of a P-MOSFET on an adjacent            active area, thereby providing an extremely compact            arrangement of the light emitting device coupled to a            switching element.    -   4. For monolithic integration with pure CMOS process on        thin-film SOI or GOI substrates, a lateral contact to the bottom        electrode is provided by not fully encircling the active area        with isolation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A presents a configuration for monolithic integration, showing oneN-MOSFET connecting to the bottom electrode of one photonic device, inwhich the “Acceleration” region is n-type and is in the bulk, while thep-type epitaxial layer is the region in which light emission throughimpact ionization takes place. The contact to the bottom electrode ismade outside the active area, through the n-Well implant.

FIG. 1B shows a configuration which differs from that of FIG. 1A, inthat the “Acceleration” region is epitaxially deposited on a highlyn-type doped surface. The “Acceleration” region can be undoped or lowlydoped.

FIG. 1C shows a configuration which differs from that of FIG. 1B, inthat the “Acceleration” region is epitaxially deposited on a highlyn-type doped surface, and an optional “energy filtering” region isplaced between the “Acceleration” and the region in which light emissionthrough impact ionization takes place. The “Acceleration” and the“Filtering” regions can be undoped or lowly doped.

FIG. 2A presents a configuration, which may or may not be monolithicallyintegrated with CMOS, made on bulk substrates, showing one photonicdevice and the contacts to the top and bottom electrodes, in which the“Acceleration” region is p-type and is in the bulk, while the n-typeepitaxial layer is the region in which light emission through impactionization takes place. The contact to the bottom electrode is madeoutside the active area, through the p-Well implant.

FIG. 2B shows a configuration which differs from that of FIG. 2A, inthat the “Acceleration” region is epitaxially deposited on a highlyn-type doped surface. The “Acceleration” region can be undoped or lowlydoped.

FIG. 2C shows a configuration which differs from that of FIG. 2B, inthat the “Acceleration” region is epitaxially deposited on a highlyn-type doped surface, and an optional “energy filtering” region isplaced between the “Acceleration” and the region in which light emissionthrough impact ionization takes place. The “Acceleration” and the“Filtering” regions can be undoped or lowly doped.

FIG. 3A presents a configuration for monolithic integration onultra-thin film SOI or GOI substrates, showing one N-MOSFET connectingto the bottom electrode of one photonic device, for front-side lightemission without a “Filter” region. The “Accelerator” region, which canbe undoped, can be deposited directly on the n-type active area,followed by the deposition of the p-type film for light emission byimpact ionization.

FIG. 3B presents a configuration for monolithic integration onultra-thin film SOI or GOI substrates, showing one N-MOSFET connectingto the bottom electrode of one photonic device, for front-side lightemission with a “Filter” region. The “Accelerator” region, which can beundoped, can be deposited directly on the n-type active area, followedby the “Filter” layers, and the deposition of the p-type film for lightemission by impact ionization.

FIG. 3C presents a configuration for monolithic integration onultra-thin film SOI or GOI substrates, showing one N-MOSFET connectingto the bottom electrode of one photonic device, for back-side lightemission without a “Filter” region. The “Accelerator” region, which canbe undoped, can be deposited directly on the n-type active area,followed by the deposition of the p-type film for light emission byimpact ionization.

FIG. 3D presents a configuration for monolithic integration onultra-thin film SOI or GOI substrates, showing one N-MOSFET connectingto the bottom electrode of one photonic device, for back-side lightemission with a “Filter” region. The “Accelerator” region, which can beundoped, can be deposited directly on the n-type active area, followedby the “Filter” layers, and the deposition of the p-type film for lightemission by impact ionization.

FIG. 4A presents a configuration, which may or may not, bemonolithically integrated with CMOS, made on ultra-thin film SOI or GOIsubstrates, showing one photonic device and the contacts to the top andbottom electrodes, for front-side light emission without a “Filter”region. The “Accelerator” region, which can be undoped, can be depositeddirectly on the n-type active area, followed by the deposition of thep-type film for light emission by impact ionization.

FIG. 4B presents a configuration, which may or may not be monolithicallyintegrated with CMOS, made on ultra-thin film SOI or GOI substrates,showing one photonic device and the contacts to the top and bottomelectrodes, for front-side light emission with a “Filter” region. The“Accelerator” region, which can be undoped, can be deposited directly onthe n-type active area, followed by the “Filter” layers, and thedeposition of the p-type film for light emission by impact ionization.

FIG. 4C presents a configuration, which may or may not be monolithicallyintegrated with CMOS, made on ultra-thin film SOI or GOI substrates,showing one photonic device and the contacts to the top and bottomelectrodes, for back-side light emission without a “Filter” region. The“Accelerator” region, which can be undoped, can be deposited directly onthe n-type active area, followed by the deposition of the p-type filmfor light emission by impact ionization.

FIG. 4D presents a configuration, which may or may not, bemonolithically integrated with CMOS, made on ultra-thin film SOI or GOIsubstrates, showing one photonic device and the contacts to the top andbottom electrodes, for back-side light emission with a “Filter” region.The “Accelerator” region, which can be undoped, can be depositeddirectly on the n-type active area, followed by the “Filter” layers, andthe deposition of the p-type film for light emission by impactionization.

FIG. 5 shows an exemplary qualitative band diagrarn of a device with ap-type silicon bottom electrode, an undoped silicon “Accelerator”region, and n-type doped Ge layer, capped by a thin silicon layer, alson-type doped.

FIG. 6 shows an exemplary qualitative band diagram of a device with ap-type silicon bottom electrode, an undoped silicon “Accelerator”region, and n-type doped (SiC)—(GeC) superlattice layers, capped by athin silicon layer, also n-type doped.

FIG. 7 shows an exemplary qualitative band diagram of a device with ap-type silicon bottom electrode, an undoped SiGe or SiGeC “Accelerator”region, and n-type doped Ge layer, capped by a thin silicon layer, alson-type doped. At the interface of the “Accelerator” region with then-type doped light emitting layer, there is a conduction band offset.That offset can be used as an energy filter, in the sense that onlycarriers with kinetic energy (along the direction perpendicular to thesubstrate) above the barrier reach the n-type doped layer. The barriershould be thin enough so that carriers cross it without scattering, butthick enough to prevent tunneling. This barrier should prevent thermalcarriers from reaching the n-type doped layer, and therefore suppressesa large portion of current that would not induce impact ionization onthe n-type doped layer.

FIG. 8 shows an exemplary qualitative band diagram of a device underforward bias, rather than reverse avalanche bias. This device shows thatwith a conduction band barrier at the n-type doped light emitter and themiddle layer (in this case not really an “accelerator”), it is possibleto keep to prevent electrons to move towards the substrate, but toinject electrons to the n-type layer. The device of FIG. 9 is meant tohave a direct bandgap n-type layer, because the recombinations in thatlayer are between thermal electrons and thermal holes.

FIGS. 9A to 9H show the most relevant steps/modules of Process Flow # 1.

FIGS. 10A to 10E show the most relevant steps/modules of Process Flow#2.

FIGS. 11A to 11F show the most relevant steps/modules of Process Flow#3.

FIGS. 12A to 12H show the most relevant steps/modules of Process Flow#4.

DETAILED DESCRIPTION OF THE INVENTION

1. Method of Fabrication

The present invention makes use of the fabrication architecture andprocess flows disclosed in WO 2002/33755 which covers the fabrication ofAvalanche Photo-Diodes (APDs) with epitaxially grown active layers,monolithically integrated with bulk CMOS devices. The fundamentaladvantage of that fabrication architecture is that some of the activelayers of the APDs are epitaxially deposited on CMOS active areas,immediately adjacent to MOSFETs, thereby resulting in a very compactmonolithic integration with CMOS. Because some of the active layers areepitaxial layers deposited on a silicon surface (active area), it ispossible to have materials that are epitaxially compatible with silicon,other than silicon itself.

Similarly to WO 2002/33755 and WO 2004/027879, the method of fabricationof the present invention, follows the standard CMOS process flow untilthe formation of silicide layers. Ideally the epitaxial deposition takesplace after all high temperature steps, such as the ion implantation andannealing of the source/drain CMOS junctions, have already taken place,so that the “as deposited” heterojunction and doping profiles are notmodified by temperature induced diffusion and/or strain relaxation. Insub-100 nm CMOS, the subsequent processing steps, such as the silicideformation, as well as all metallization steps, can be performed attemperatures lower than those used for the epitaxial growth.

The method of fabrication of the present invention enables a verycompact monolithic integration of APDs/ALEDs with advanced CMOS to forman “Active Matrix” of Pixels/Lixels, in which the APDs/ALEDs compriseepitaxially grown active layers with sophisticated doping andheterojunction profile engineering. This permits the fabrication ofactive regions with profoundly modified band structure, resulting ingreatly improved optoelectronic characteristics with respect to thesubstrate (silicon or germanium, or a relaxed SiGe buffer layer). Itmust be emphasized that such sophisticated doping and heterojunctionprofile engineering are impossible with devices made without theepitaxial growth of those layers.

WO 2002/33755 provides the method of fabrication on bulk substrates andthick-film SOI. WO 2004/027879 provided the method of fabrication onthin-film SOI or GOI substrates, for front and/or back-sideillumination. The present disclosure is applicable to bulk, thick-filmand thin-film SOI or GOI substrates.

It should be noticed that the optimal device layers for light emissioncan also be fabricated on substrates without CMOS devices, resulting isa far simpler and lower cost process flow. Some applications, such asSolid State Lighting (SSL), do not require the monolithic integration ofthe the advanced epitaxial layers with CMOS. In this case there arethree options: (1) fabricate single large ALED/Lixel device, (2)fabricate 2-dimensional array of ALED/Lixel devices contacted inparallel, (3) fabricate 2-dimensional array of ALED/Lixel devicescontacted individually through “passive matrix addressing”.

The process flow disclosed in WO 2002/33755 is an example in which it isassumed that the thermal budget required for the epitaxial deposition ofthe SiGeC layers is sufficiently low to be performed after the formationof the highly-doped source/drain regions for deep submicron (e.g. 0.18μm) CMOS technologies. Low temperature epitaxial deposition processes(including the pre-epitaxy surface preparation) have been demonstrated.However, the most commonly used processes in production still requiresomewhat larger thermal budgets to achieve the required films.

If the thermal budget for the epitaxial deposition of the SiGeC film isconsidered too high, then the deposition step could be inserted beforethe formation of the source/drain junctions. The thermal budget forepitaxial deposition of SiGeC films has been decreasing over time. It iswidely acknowledged that there is a tendency towards lower thermalbudgets, for this and many other process steps. It is reasonable toexpect that at some time in the near future, production grade equipmentand recipes will be compatible with the insertion of the epitaxialdeposition step after the formation of the source/drain regions.Changing the substrate from silicon to germanium for CMOS technologiesbelow 45 nm would farther help decreasing the thermal budget because thegermanium native oxide is very easily removed (it is water soluble),compared to the very stable silicon oxide. Therefore it is to beexpected that germanium substrates/surfaces will enable multipleepitaxial deposition steps without impacting pre-existingdoping/heterojunction profiles.

In the process flows described below, the “Isolation Module” can beconventional LOCOS or STI technologies, but STI will be the preferredone. Also, the “Ion Implantation Modules”, as well as the “SilicideModules”, refer to the respective conventional processmodules/steps/recipes. In the following process flows, the details ofmaterials, doping and heterojunction profiles, are not shown, becausethey are not needed for the description of the process flow, and becausethe process flow does not change if the details of those layers arechanged.

The following is the list of layers/materials referenced in the Figures:

-   100—p-substrate-   101—Active-   102—STI-   103—p-well-   104—n-well-   105—p-type doped regions (105), isolating adjacent photo-diode    active areas-   106—n-type doped active area for epitaxial layers of APDs/ALEDs.-   107—Gate Insulator of MOSFET-   108—NMOS LDDs-   109—Highly n-type doped region, such as NMOS Source/Drain regions    (HDD)-   110—Field Isolation of Thin-Film SOI-   111—Gate electrodes-   112—Nitride spacers-   113—Deep Trench Isolation (DTI)-   114—Nitride film as hard mask for epitaxy-   115—Silicide-   116—Pre-Metal Dielectric-   117—Epitaxially deposited films—single crystal material over active    areas-   118—Epitaxially deposited films—amorphous/poly-crystalline material    over field isolation areas-   119—N-type implant into active areas of APD-   120—Buried oxide of SOI substrates-   121—SOI mechanical substrate-   126—Light blocking layer-   127—Red color filter-   128—Green color filter-   129—Blue Color filter-   130—Contacts-   131—Metal-1-   150—Transparent substrate-   151—N-type silicon substrate-   152—Back-side metallization-   160—P-type electrode and light emitting layer-   161—N-type electrode light emitting layer-   162—Acceleration region for front-side light emission-   163—Acceleration region for back-side light emission-   164—Filtering region for front-side light emission-   165—Filtering region for back-side light emission-   166—P-type electrode-   167—N-type electrode

Exemplary Process Flow #1

Fabrication of multiple ALED/Lixel devices contacted individuallythrough “active matrix addressing”, made on a bulk substrate with aprocess flow for monolithic integration with CMOS devices. Thisexemplary process flow makes use of a twin-well process on a p-substratesilicon wafer. The flow described below indicates only the mostimportant process modules.

Sequence of Process Modules (FIGS. 9A to 9H):

-   “Isolation Module”-   “P-Well Implant Module”-   “N-Well Implant Module” (FIG. 9A)-   “Poly-Gate Module” (FIG. 9B)-   “NMOS LDD Implant Module”-   “PMOS LDD Implant Module”-   “Lixel Implant Module” (FIG. 9C)-   “Nitride Spacer Module”-   “NMOS IDD Implant Module”-   “NMOS HDD Implant Module” (FIG. 9D)-   “Pre-Epitaxy Module” (FIG. 9E)    -   A. Deposition of Si₃N₄ to be used as a hard mask for epitaxy;    -   B. Photolithography defining the windows were the epitaxial        films are to be grown;    -   C. Etch to open windows over the active areas onto which        epitaxial layers will be grown;    -   D. Photoresist strip and clean;

“Epitax Module” (FIG. 9F)

-   -   A. Pre-epitaxy clean;    -   B. Epitaxial growth of the layers with optimized doping and        heterojunction profiles. The epitaxial growth can be selective        or non-selective. The figure shows a non-selective growth;    -   C. Photolithography defining where the epitaxial films are to be        removed;    -   D. Etch to remove epitaxial layers, stopping on the Si₃N₄ film        underneath;    -   E. Photoresist strip and clean;

“Silicide Module” (FIG. 9G)

-   -   Formation of silicide with conventional methods/recipes:    -   A. Deposition of Si₃N₄ to be used as a hard mask for silicide        formation;    -   B. Photolithography defining the windows where a silicide is to        be formed;    -   C. Etch to open windows over the active areas in which a        silicide will be formed;    -   D. Photoresist strip and clean;    -   E. Deposition (for example by sputtering) of a metal film;    -   F. Thermal annealing to form a silicide    -   G. Removal (for example, selective wet etch) of unreacted metal;

“Metallization Module” (FIG. 9H)

-   -   The metallization technology to be used can be the same used in        standard CMOS technology.    -   The thickness of the epitaxial layers, if it exceeds the typical        thickness of the poly silicon gates of a given CMOS technology,        might require some fine tuning of the planarization before        forming contacts and metal-1 lines.

Exemplary Process Flow #2

Fabrication of multiple ALED/Lixels devices contacted individuallythrough “passive matrix addressing”, made on a bulk substrate with aprocess flow simpler than the one required for monolithic integrationwith CMOS devices. This exemplary process flow makes use of p-substratesilicon wafers, and skips many steps used to fabricate standard CMOSdevices.

Sequence of Process Modules (FIGS. 10A to 10E):

“Isolation Module”

“Ion Implantation Module” (FIG. 10A)

-   -   This module defines the locations to be ion implanted with        n-type dopants.    -   In a preferred embodiment, there are three separate patterned        ion implantation steps.    -   A first implant is the standard N-Well implant in CMOS        processes, overlapping active areas and isolation regions.    -   A second implant into the active areas, will create doping        levels similar to those used at the collector region of        high-speed HBT devices in BiCMOS process. The second implant        overlaps the N-Well implant along pre-determined regions.    -   A third implant overlaps the N-Well implant along pre-determined        regions, and provides high dopant concentration in pre-defined        surface regions.

“Pre-Epitaxy Module” (FIG. 10B)

-   -   A. Deposition of Si₃N₄ to be used as a hard mask for epitaxy;    -   B. Photolithography defining the windows where the epitaxial        films are to be grown;    -   C. Etch to open windows over the active areas onto which        epitaxial layers will be grown;    -   D. Photoresist strip and clean;

“Epitax Module” (FIG. 10C)

-   -   A. Pre-epitaxy clean;    -   B. Epitaxial growth of the layers with optimized doping and        heterojunction profiles. The epitaxial growth can be selective        or non-selective. The figure shows a non-selective growth;    -   C. Photolithography defining where the epitaxial films are to be        removed;    -   D. Etch to remove epitaxial layers, stopping on the Si₃N₄ film        underneath;    -   E. Photoresist strip and clean;

“Silicide Module” (FIG. 10D)

-   -   Formation of silicide with conventional methods/recipes:    -   A. Deposition of Si₃N₄ to be used as a hard mask for silicide        formation;    -   B. Photolithography defining the windows where a silicide is to        be formed;    -   C. Etch to open windows over the active areas in which a        silicide will be formed;    -   D. Photoresist strip and clean;    -   E. Deposition (for example by sputtering) of a metal film;    -   F. Thermal annealing to form a silicide    -   G. Removal (for example, selective wet etch) of unreacted metal;

“Metallization Module” (FIG. 10E)

-   -   The metallization technology to be used can be the same used in        standard CMOS technology.    -   The thickness of the epitaxial layers, if it exceeds the typical        thickness of the poly silicon gates of a given CMOS technology,        might require some fine tuning of the planarization before        forming contacts and metal-1 lines.

Exemplary Process Flow #3

Fabrication of multiple ALED/Lixel devices, all contacted simultaneouslyin parallel, made on a bulk substrate with a process flow simpler thanthe one required for monolithic integration with CMOS devices. Thisexemplary process flow makes use of n-substrate silicon wafers, andskips many steps used to fabricate standard CMOS devices. With this flowthe contact to the bottom electrode is made through the back-side of thesubstrate, and that is why n-type substrate is used.

Sequence of Process Modules (FIGS. 11A to 11F):

“Isolation Module”

“Ion Implantation Module” (FIG. 11A)

-   -   With a n-type substrate, and contact to the bottom electrode        being made through the back-side of the (thinned) wafer, there        is no need for N-Well implant, nor for an implant to produce        high n-type dopant concentration near the surface of the wafer.    -   Implant into the active areas, to create doping levels similar        to those used at the collector region of high-speed HBT devices        in BiCMOS process. The mask for this patterned implant overlaps        the active areas in order to leave lower doping regions around        the isolation regions.

“Pre-Epitaxy Module” (FIG. 11B)

-   -   A. Deposition of Si₃N₄ to be used as a hard mask for epitaxy;    -   B. Photolithography defining the windows where the epitaxial        films are to be grown;    -   C. Etch to open windows over the active areas onto which        epitaxial layers will be grown;    -   D. Photoresist strip and clean;

“Epitaxy Module” (FIG. 11C)

-   -   A. Pre-epitaxy clean;    -   B. Epitaxial growth of the layers with optimized doping and        heterojunction profiles. The epitaxial growth can be selective        or non-selective. The figure shows a non-selective growth;    -   C. Photolithography defining where the epitaxial films are to be        removed;    -   D. Etch to remove epitaxial layers, stopping on the Si₃N₄ film        underneath;    -   E. Photoresist strip and clean;

“Suicide Module” (FIG. 11D)

-   -   Formation of silicide with conventional methods/recipes:    -   A. Deposition of Si₃N₄ to be used as a hard mask for silicide        formation;    -   B. Photolithography defining the windows where a silicide is to        be formed;    -   C. Etch to open windows over the active areas in which a        silicide will be formed;    -   D. Photoresist strip and clean;    -   E. Deposition (for example by sputtering) of a metal film;    -   F. Thermal annealing to form a silicide    -   G. Removal (for example, selective wet etch) of unreacted metal;

“Metallization Module” (FIG. 11E)

-   -   The metallization technology to be used can be the same used in        standard CMOS technology.    -   The thickness of the epitaxial layers, if it exceeds the typical        thickness of the poly silicon gates of a given CMOS technology,        might require some fine tuning of the planarization before        forming contacts and metal-1 lines. In a typical application,        the metallization module will have just one metal level.

“Back-Side Module” (FIG. 11F)

-   -   Deposition and annealing of a metal (for example aluminum) on        the back-side of the wafer, to form the contact to the bottom        electrode of the devices fabricated on the front-side of the        wafer. Prior to the deposition of the metal on the back-side,        the wafer is thinned to minimize the series resistance to the        structures fabricated on the front side.

Exemplary Process Flow #4

Fabrication of multiple ALED/Lixels devices contacted individuallythrough “active matrix addressing”, made on a Thin-Film SOI (or GOI)substrate with a process flow for monolithic integration with CMOSdevices. The flow described below indicates only the most importantprocess modules.

Sequence of Process Modules (FIGS. 12A to 12H):

“Isolation Module” FIG. 12A)

“Poly-Gate Module” & “NMOS SID Implant Module” (FIG. 12B)

“Pre-Epitaxy Module” (FIG. 12C)

-   -   A. Deposition of Si₃N₄ to be used as a hard mask for epitaxy;    -   B. Photolithography defining the windows where the epitaxial        films are to be grown;    -   C. Etch to open windows over the active areas onto which        epitaxial layers will be grown;    -   D. Photoresist strip and clean;

“Epitaxy Module” (FIG. 12D)

-   -   A. Pre-epitaxy clean;    -   B. Epitaxial growth of the layers with optimized doping and        heterojunction profiles. The epitaxial growth can be selective        or non-selective. The figure shows a non-selective growth;    -   C. Photolithography defining where the epitaxial films are to be        removed;    -   D. Etch to remove epitaxial layers, stopping on the Si₃N₄ film        underneath;    -   E. Photoresist strip and clean;

“Silicide Module” (FIG. 12E)

-   -   Formation of silicide with conventional methods/recipes:    -   A. Deposition of Si₃N₄ to be used as a hard mask for silicide        formation;    -   B. Photolithography defining the windows where a suicide is to        be formed;    -   C. Etch to open windows over the active areas in which a        silicide will be formed;    -   D. Photoresist strip and clean;    -   E. Deposition (for example by sputtering) of a metal film;    -   F. Thermal annealing to form a silicide    -   G. Removal (for example, selective wet etch) of unreacted metal;

“Metallization Module” (FIG. 12F)

-   -   The metallization technology to be used can be the same used in        standard CMOS technology.    -   The thickness of the epitaxial layers, if it exceeds the typical        thickness of the poly silicon gates of a given CMOS technology,        might require some fine tuning of the planarization before        forming contacts and metal-1 lines.

Optional “Backside Processing Module” FIG. 12G)

-   -   After all processing has been done on the front-side of the        substrate, there is an option to remove the back-side of the        substrate. The buried oxide provides a marking layer for        whatever method of removing the back-side is applied. Then the        fully processed SOI (or GOD layer can either:    -   (1) Be directly bonded to a new substrate, which can be light        transparent or opaque, and can be an insulator or a conductor;    -   (2) Undergo processing on the newly exposed surface of the        buried oxide, and then bonded to a new substrate.    -   Processing on the newly exposed surface of the buried oxide can        be used for the purpose of:    -   (A) Electrical connections of structures made on the front-side        of the wafer,    -   (B) Fabrication of additional electrical, and/or electronic,        and/or optical, optoelectronic devices. Examples of such devices        can be Antennas, vertical and/or horizontal Optical (half-)        Cavities, Surface Plasmon-Polariton (SPP) structures such as        “Light Funnels” (WO 2004/027879), etc. It should be noticed that        the fabrication of an optical half-cavity on the back-side could        be complemented with the fabrication of an half-cavity on the        front side, immediately after the “Epitaxy Module”, and before        the “Silicide Module”.

2. Types of Substrates and Epitaxial Layers

The method of fabrication of the present invention, can be implementedfor different substrate materials (e.g., bulk silicon or germanium,Thick-Film SOI or GOI, Thin-Film SOI or GOI) and different orientations(<100>, <111>, etc.). Naturally there are differences with respect tothe epitaxial films that can be grown on different materials andorientations, with consequences on the optoelectronic properties, andtherefore on the performance and functionality.

It has been demonstrated that patterned areas of silicon substrates canbe processed to achieve mono-layer flatness as demonstrated by S.Tanaka, G. C. Umbach, J. M. Blakely, R. M. Tromp, M. Mankos,“Fabrication of arrays of large step-free regions on Si(001)”, Appl.Phys. Lett, Vol. 69, No. 9, pp. 1235, 26 Aug. 1996, and D. Lee, J.Blakely, “Formation and stability of large step-free areas on Si(001)and Si(111), Surf. Sci. Vol. 445, pp. 32, 2000; which is the idealsurface for the epitaxial growth of high quality pseudomorphic randomalloys and short period superlattices. The processing required toproduce atomic mono-layer flat active areas is compatible with theprocess flows of WO 2002/33755 and WO 2004/027879, as well as thelayouts of a co-pending application, so that the photonic active layersof the devices in the current disclosure can be fabricated on suchsurfaces.

Independently of substrate material and orientation, the ALEDs require acertain functionality from the active layers: a first electrode, an“acceleration” region, an optional “energy filtering region”, and an“impact ionization” or “avalanching” region which is also the secondelectrode. For example, for a device made on a n-well on a p-substrate,the bottom electrode is n-type, and the top electrode, is p-type. The“acceleration” and “energy filtering” regions can be undoped.

From these requirements it can be immediately inferred that devices madeon thin-film SOI or GOI must have the “acceleration” to be a part of theepitaxial stack, while for devices made on bulk or thick-film SOIsubstrates, carriers can be accelerated in the substrate. With bulk orthick-film SOI substrates the “acceleration” region can also beepitaxially grown, which enables bandgap engineering through carefullydesigned heterojunction profiles, with many potential advantages over an“acceleration” region made in a single homogeneous material.

An “energy filtering” region for electrons and/or holes can be made witha superlattices, as shown by J. Martorell , D. W. L. Sprung and G. V.Morozov, “Design of electron band pass filters for electrically biasedfinite superlattices”, Phys. Rev. B 69, 115309, 2004. The purpose ofbuilding such layers into the epitaxial stack, just before the formationof the “avalanching” region (top electrode), is to restrict the flux ofcarriers into the “avalanching” layer, to only those with energy withina certain range. That energy range is chosen to be the one that providesthe highest probability of generating a radiative transition inside the“avalanching” region. As a consequence, the flux of carriers with energyoutside the ideal range is suppressed, thereby sharply decreasing thetotal current, thus decreasing the total power dissipation, andtherefore significantly increasing the overall efficiency powerefficiency.

Ideally, the “energy filtering” region performs in such away that everysingle carrier that is able to cross it, causes an impact ionizationevent that results in the emission of a photon. In such a scenario, theefficiency of avalanche light emission from an indirect bandgap materialwould approach the efficiency of light emission by the recombination ofthermal carriers in direct bandgap materials.

Thin-Film SOI or GOI substrates allow the design of ALEDs for back-sideemission of photons with energy larger than the bandgap of the substrate(e.g., silicon or germanium). The buried oxide layer is a perfectmarking layer for the removal of the silicon or germanium mechanicalsubstrates, thus enabling processing directly on the back-side of theburied oxide, and subsequent bonding to a transparent substrate. One ofthe many possibilities enabled by back-side processing is thefabrication of optical cavities. This is particularly useful forvertical cavity emitting devices.

A conventional resonant optical cavity would typically require thefabrication of half-cavity on the front-side, and another half-cavity ofthe back-side of the light emitting layer. Newer concepts, employingSurface Plasmon Polaritons (SPPs), which require the fabrication ofpatterned thin films of noble metals that are not compatible withconventional CMOS layouts and/or processing, can be easily implementedon the back-side of these substrates, after finishing allCMOS-compatible processing on the front-side, as already disclosed in WO2004/027879.

FIG. 1A presents a configuration for monolithic integration, showing oneN-MOSFET connecting to the bottom electrode of one photonic device, inwhich the “Acceleration” region is n-type and is in the bulk, while thep-type epitaxial layer is the region in which light emission throughimpact ionization takes place. The contact to the bottom electrode ismade outside the active area, through the n-Well implant.

FIG. 1B shows a configuration which differs from that of FIG. 1A, inthat the “Acceleration” region is epitaxially deposited on a highlyn-type doped surface. The “Acceleration” region can be undoped or lowlydoped.

FIG. 1C shows a configuration which differs from that of FIG. 1B, inthat the “Acceleration” region is epitaxially deposited on a highlyn-type doped surface, and an optional “energy filtering” region isplaced between the “Acceleration” and the region in which light emissionthrough impact ionization takes place. The “Acceleration” and the“Filtering” regions can be undoped or lowly doped.

FIG. 2A presents a configuration, which may or may not be monolithicallyintegrated with CMOS, made on bulk substrates, showing one photonicdevice and the contacts to the top and bottom electrodes, in which the“Acceleration” region is p-type and is in the bulk, while the n-typeepitaxial layer is the region in which light emission through impactionization takes place. The contact to the bottom electrode is madeoutside the active area, through the p-Well implant.

FIG. 2B shows a configuration which differs from that of FIG. 2A, inthat the “Acceleration” region is epitaxially deposited on a highlyn-type doped surface. The “Acceleration” region can be undoped or lowlydoped.

FIG. 2C shows a configuration which differs from that of FIG. 2B, inthat the “Acceleration” region is epitaxially deposited on a highlyn-type doped surface, and an optional “energy filtering” region isplaced between the “Acceleration” and the region in which light emissionthrough impact ionization takes place. The “Acceleration” and the“Filtering” regions can be undoped or lowly doped.

FIG. 3A presents a configuration for monolithic integration onultra-thin film SOI or GOI substrates, showing one N-MOSFET connectingto the bottom electrode of one photonic device, for front-side lightemission without a “Filter” region. The “Accelerator” region, which canbe undoped, can be deposited directly on the n-type active area,followed by the deposition of the p-type film for light emission byimpact ionization.

FIG. 3B presents a configuration for monolithic integration onultra-thin film SOI or GOI substrates, showing one N-MOSFET connectingto the bottom electrode of one photonic device, for front-side lightemission with a “Filter” region. The “Accelerator” region, which can beundoped, can be deposited directly on the n-type active area, followedby the “Filter” layers, and the deposition of the p-type film for lightemission by impact ionization.

FIG. 3C presents a configuration for monolithic integration onultra-thin film SOI or GOI substrates, showing one N-MOSFET connectingto the bottom electrode of one photonic device, for back-side lightemission without a “Filter” region. The “Accelerator” region, which canbe undoped, can be deposited directly on the n-type active area,followed by the deposition of the p-type film for light emission byimpact ionization.

FIG. 3D presents a configuration for monolithic integration onultra-thin film SOI or GOI substrates, showing one N-MOSFET connectingto the bottom electrode of one photonic device, for back-side lightemission with a “Filter” region. The “Accelerator” region, which can beundoped, can be deposited directly on the n-type active area, followedby the “Filter” layers, and the deposition of the p-type film for lightemission by impact ionization.

FIG. 4A presents a configuration, which may or may not, bemonolithically integrated with CMOS, made on ultra-thin film SOI or GOIsubstrates, showing one photonic device and the contacts to the top andbottom electrodes, for front-side light emission without a “Filter”region. The “Accelerator” region, which can be undoped, can be depositeddirectly on the n-type active area, followed by the deposition of thep-type film for light emission by impact ionization.

FIG. 4B presents a configuration, which may or may not be monolithicallyintegrated with CMOS, made on ultra-thin film SOI or GOI substrates,showing one photonic device and the contacts to the top and bottomelectrodes, for front-side light emission with a “Filter” region. The“Accelerator” region, which can be undoped, can be deposited directly onthe n-type active area, followed by the “Filter” layers, and thedeposition of the p-type film for light emission by impact ionization.

FIG. 4C presents a configuration, which may or may not be monolithicallyintegrated with CMOS, made on ultra-thin film SOI or GOI substrates,showing one photonic device and the contacts to the top and bottomelectrodes, for back-side light emission without a “Filter” region. The“Accelerator” region, which can be undoped, can be deposited directly onthe n-type active area, followed by the deposition of the p-type filmfor light emission by impact ionization.

FIG. 4D presents a configuration, which may or may not, bemonolithically integrated with CMOS, made on ultra-thin film SOI or GOIsubstrates, showing one photonic device and the contacts to the top andbottom electrodes, for back-side light emission with a “Filter” region.The “Accelerator” region, which can be undoped, can be depositeddirectly on the n-type active area, followed by the “Filter” layers, andthe deposition of the p-type film for light emission by impactionization.

3. Device “Active Layers”

As already described in the previous section, there are several groupsof “active layers”. The bottom electrode, the “acceleration” region, theoptional “energy filtering” region, and the “avalanching” region, whichis the region in which light emission through impact ionization takesplace, and that can be simultaneously the top electrode.

It is in the “acceleration” region that thermal carriers gain thenecessary energy to successfully cause impact ionization in an adjacent“avalanching” region. The ability to gain energy from the electric fieldin the “acceleration” region depends on the electron and/or holemobility in that region: the higher the mobility the larger the numberof carriers acquiring the desired energy level to cause impactionization. Therefore, it is highly advantageous for the “acceleration”region to be an undoped or lowly doped, single-crystalline region. Theacceleration region can be designed to be part of the substrate of bulkwafers or thick-film SOI substrates, or can be epitaxially grown on anytype of substrate: bulk, thick-film SOI, or thin-film SOI or GOI. Withan epitaxially grown “acceleration” region, it is also possible to havesophisticated heterojunction and or doping profiles to enhanceperformance and/or functionality.

The optional “energy filtering” region, if implemented with asuperlattice, requires single crystal epitaxial films withheterojunction engineering at the mono-layer level. Therefore thisoptional set of device layers must be epitaxially grown, regardless ofthe type of substrate. In a preferred implementation, the “avalanching”region is also a single crystalline film with sophisticatedheterojunction and doping profiles. However, it may also be anon-pseudomorphic film (alloys and/or superlattices), such aspoly-crystalline, nano-crystalline, amorphous, or porous, of any of thematerials listed below, and that can be formed on the silicon orgermanium substrates.

The following subsections of the present disclosure provide moredetailed information about different classes of materials/layers thatcan be used as the light emitting layer.

3.1. Epitaxial Layers using only Group V elements (C. Si, Ge, Sn) on SiSubstrates

For reasons mentioned earlier, an exemplary implementation formonolithic integration with current state of the art CMOS technology isstraightforward with random alloys and/or superlattices of pseudomorphicSi_(1-x)Ge_(x), and/or Si_(1-y)C_(y), and/or Si_(1-x-y)Ge_(1-x)C_(y),and/or Ge_(1-x)C_(x), strained to Si substrates, with any of the morerelevant crystal orientations, such as (100), (111), or (311) forexample.

The integration of random alloys and/or superlattices of pseudomorphicSi_(1-x)Ge_(x), and/or Si_(1-y)C_(y), and/or Si_(1-x-y)Ge_(x)C_(y), orand/or Ge_(1-z)C_(z), strained to Si substrates, and their incorporationinto a device design in which light emission due to avalanche takesplace inside those films/materials, enables performance andfunctionality gains that are more significant than a slight improvementupon pure silicon devices. The reason for a qualitative jump inperformance, such as increased efficiency of radiative transitions, aswell as functionality by the selection of preferential range ofwavelengths, is related to the qualitative differences between the bandstructure of silicon and germanium, and their combinations into randomalloys and superlattices.

While in silicon the difference in energy between the indirect bandgap(1.1 eV) and the lowest direct bandgap (3.2 eV) is 2.1 eV, in germaniumthe difference between the indirect bandgap (0.66 eV) and the smallestdirect bandgap (0.8 eV) is only 0.14 eV. For germanium, it is onlynecessary to add 0.14 eV of kinetic energy to the thermal electrons inthe L-valley (along the <111> direction) to enable them to move into theΓ-valley, and thus enable highly efficient direct radiative transitions,by emitting 1.55 μm wavelength (0.8 eV) photons. Therefore, a region inwhich light emission by avalanching takes place, that is made ofgermanium rather than silicon, still has an indirect bandgap, but theconsequences for optoelectronic intern and transitions are more complexthan simply taking the physical picture of the indirect bandgap ofsilicon and reduce it from 1.1 eV to 0.66 eV.

For the compositions of SiGe and/or SiGeC random alloys, used withcurrent technologies, the bandgap structure is very similar to that ofsilicon's, including the big difference between the indirect andsmallest direct bandgaps. However, that is not the case with SiGe/Sisuperlattices, in which “zone folding” radically changes the bandstructure along the axis of growth of the superlattice, as explained byM. J. Shaw and M. Jaros: “Fundamental Physics of Strained layer GeSi:Quo Vadis”, Chapter 4 of “Germanium Silicon: Physics and Materials”,Vol. 56, Academic Press, 1999. The impact of strain, and the impact ofthe superlattice period on the band structure have been theoreticallyand experimentally studied for the purpose of light detection andemission.

Some of the most promising superlattices seem to be Si₅—Ge₅ strained toa virtual substrate of relaxed Si_(0.5)Ge_(0.5) random alloy. Thisparticular superlattice is not suitable for monolithic integration withCMOS due to the required virtual substrate. Other superlattice designsthat may result in a similar direct bandgap structure can be formeddirectly on the silicon substrate. One such superlattice consists offive mono-layers of Si_(1-y)C_(y) random alloy, with a large percentageof carbon, alternating with five mono-layers of pure Ge or Ge_(1-z)C_(z)random alloy. The amount of carbon in the Ge_(1-z)C_(z) layer can bevaried depending on the goals to be achieved. Ge_(1-z)C_(z) alloysstrained to Si have been demonstrated by M. Todd, J. Kouvetakis, D. J.Smith, “Synthesis and characterization of heteroepitaxialdiamond-structured Ge_(1-x)C_(x) (x=1.5-5.0%) alloys using chemicalvapor deposition”, Appl. Phys. Lett., Vol. 68, No. 17, 22 Apr. 1996, pp.2407-2409; for much higher Carbon content, and for much thicker than theones envisaged as required for this superlattice.

The effect of carbon on the band-structure of Germanium strained to thesilicon lattice is not well characterized. What follows assumes that theaddition of small amounts of carbon to a pure germanium film, will notresult in qualitative changes, nor in dramatic quantitative changes tothe band structure of the germanium film. It is also assumed that verysmall amounts of carbon can increase the critical thickness of nearlypure germanium layers strained to silicon substrates of any of thetechnologically relevant crystalline orientations.

Estimations indicate that the critical thickness for pure Ge on Si(100)is 1.2 nm. The incorporation of small amounts of carbon into the Gelayer can increase its critical thickness and enable he fabrication ofSLs with larger number of monolayers of Ge (actually Ge_(1-z)C_(z)).Depending on the carbon content of both components of the(Si_(1-y)C_(y))₅—(Ge_(1-z)C_(z))₅ superlattice, it is possible for theoverall layer stack to be strain compensated. It is possible to make thecompressive strain in the (Si_(1-y)C_(y))₅ layer to be even larger thanthe tensile strain in the (Ge_(1-z)C_(z))₅ layer.

To have a pseudo-direct bandgap miniband, it is desirable to have asplit in the conduction band of the silicon-rich layer—see for exampleF. Cerdeira: “Optical Properties”, Chapter 5, page 231 of “GermaniumSilicon: Physics and Materials”, Vol. 56, Academic Press, 1999. Sitensile strained layers can be accomplished by growing silicon on arelaxed SiGe buffer layer (virtual substrate), or by including carbon inthe layer: Si_(1-y)C_(y) strained to a Si substrate, as demonstrated byK. Eberl, K. Brunner, O. G. Schmidt, “Si_(1-y)C_(y) andSi_(1-x-y)Ge_(x)C_(y) Alloy Layers” Chapter 8 of “Germanium Silicon:Physics and Materials”, Vol. 56, Academic Press, 1999. See FIG. 2 (pp.389) and 13 (pp. 403) of same book. FIGS. 16 (pp. 406) and 17 (pp. 407)show the band edge and bandgap of Si_(1-y)C_(y) strained to Si.

In the Si₅—Ge₅ superlattice strained to a virtual substrate of relaxedSi_(0.5)Ge_(0.5) random alloy, that split is induced by the tensilestrain, which causes the four in-plane Δ-valleys (Δ_(∥)) to be lifted(with respect to a bulk substrate), leaving unchanged the two Δ-valleys(Δ_(⊥)) along the direction of growth of the superlattice. In the(Si_(1-y)C_(y))₅—(Ge_(1-z)C_(z)) superlattice strained to the siliconlattice, the split in the conduction band of the silicon-rich layer isinduced by the compressive strain caused by the presence of carbon inthat film. The compressive strain causes the two Δ-valleys (Δ_(⊥)) alongthe direction of growth of the superlattice to be lowered with respectto a bulk substrate and the in-plane four Δ-valleys (Δ_(∥)).

The (Si_(1-y)C_(y))₅—(Ge_(1-z)C_(z))₅ superlattice is likely to have avery large “oscillator strength”, similar to that of Si₅—Ge₅ strained toa relaxed Si_(0.5)Ge_(0.5) random alloy. The(S_(i-y)C_(y))₅—(Ge_(1-z)C_(z))₅ superlattice does not have to be straincompensated, but if it were, its total thickness would not be limited bystrain.

Theory predicts that the top of the valence band of the SL originates inthe bulk-Ge Γ-point states. Therefore replacing pure Ge withGe_(1-z)C_(z) should increase the bandgap of the SL, because reductionin the strain of the Ge layer results in a smaller offset in the valenceband with respect to the Si bulk. Therefore, there are good reasons toexpect that a (Ge_(1-z)C_(z))_(m)—(Si_(1-y)C_(y))_(n) superlattice (forexample with m=n=5) strained to a silicon substrate, should result in apseudo-direct bandgap, with a high oscillator strength. The amount ofcarbon in the Si-rich layer controls the conduction band edge of the SL,and the amount of carbon in the Ge-rich layer controls the valence bandedge of the SL. Therefore, through the modulation of the carbon contentin both layers of the SL, it is possible to perform bandgap engineeringof the SL layers.

The fact that the SL conduction band edge is lower in the direction ofgrowth than laterally may have profound consequences for electrontransport: the conduction band edge (potential energy) is lower for thedirection perpendicular to the substrate, than for the directionsparallel to the substrate. This anisotropy is likely to have veryimportant consequences for scattering events, such as impact ionization,in particular for the angular distribution of the velocity of thegenerated electron-hole pairs.

The probability of recombination across the direct bandgaps in any ofthese materials can be further increased by heavy n-type doping in theselayers. One reason is that the lowest states in the conduction band areoccupied by electrons from the heavy doping, thus pushing theFermi-Level towards the direct badgap edge of the conduction band. Thisis especially relevant to indirect bandgap films/materials in which saiddirect bandgap is not much larger than the lowest indirect bandgap, suchas germanium and superlattices containing Si, Ge, C, in which thepseudo-direct bandgap is only slightly larger than the lowest indirectbandgap.

The highest in-situ doping level possible might be achieved byincorporating impurities of several species, rather than just one as itis normally done. For example the epitaxial of Si, SiGe, SiGeC, Ge filmsprocess could have gases carrying the three commonly used n-typedopants: P, As, Sb. Very heavy doping means that there is a significantpercentage of foreign atoms in the crystalline lattice. Germanium atomconcentration is 4.42×10²² cm⁻³. A doping concentration of for example,5×10² 1 cm⁻³ represents more than 1% of the atoms in the lattice. Itshould be kept in mind that in many SiGeC films, the carbonconcentration is below 1%, and still there are important chemical andstrain effects from carbon in those films. Another consequence of heavydoping is “Band Gap Narrowing” (BGN). This effect is detrimental forcertain devices such as solar cells, but it can be an advantage for thedevices of the present invention.

In conventional homojunction or heterojunction interband light emittingdiodes, radiative recombination takes place in a region separating ap-type and n-type regions that act as hole and electron injectors,respectively. In homojunction devices this region is simply thedepletion region. In heterojunction devices this region is chosen tohave a narrower bandgap, and have a type-II alignment with respect tothe p-type and n-type doped regions, thereby confirming electrons andholes.

In the present invention some implementations depart from thisconventional device architecture, and the region in which radiativerecombination is to take place is the heavily n-type doped region.

FIGS. 5, 6, 7, and 8 show device layer profiles, in which the doping andheterojunction profiles are such that light emission through impactionization takes place in a highly n-type doped region.

FIG. 5 shows a qualitative band diagram of a device with a p-typesilicon bottom electrode, an undoped silicon “Accelerator” region, andn-type doped Ge layer, capped by a thin silicon layer, also n-typedoped.

FIG. 6 shows a qualitative band diagram of a device with a p-typesilicon bottom electrode, an undoped silicon “Accelerator” region, andn-type doped (SiC)—(GeC) superlattice layers, capped by a thin siliconlayer, also n-type doped.

FIG. 7 shows a qualitative band diagram of a device with a p-typesilicon bottom electrode, an undoped SiGe or SiGeC “Accelerator” region,and n-type doped Ge layer, capped by a thin silicon layer, also n-typedoped. At the interface of the “Accelerator” region with the n-typedoped light emitting layer, there is a conduction band offset. Thatoffset can be used as an energy filter, in the sense that only carrierswith kinetic energy (along the direction perpendicular to the substrate)above the barrier reach the n-type doped layer. The barrier should bethin enough so that carriers cross it without scattering, but thickenough to prevent tunneling. This barrier should prevent thermalcarriers from reaching the n-type doped layer, and therefore suppressesa large portion of current that would not induce impact ionization onthe n-type doped layer.

In the devices of FIGS. 5, 6, and 7, free holes are injected into theheavy n-type doped region Electron-hole pair generation by impactionization inside the n-type region. This requires the existence of an“acceleration” region in which carriers gain energy from an electricfield.

Since in silicon and germanium electron mobility is significantly higherthan hole mobility, it is advantageous to design devices in which impactionization is caused by hot electrons, rather than hot holes. Therefore,devices should be designed for light emission through avalanche of hotelectrons. However in superlattice acceleration regions this may not bethe case, and it may happen that it is more advantageous to have lightemission from impact ionization by hot holes.

FIG. 8 shows a qualitative band diagram of a device under forward bias,rather than reverse avalanche bias. This device shows that with aconduction band barrier at the interface between the n-type doped lightemitter and the middle layer, which in this case not really an“accelerator”, it is possible to prevent electrons from moving towardsthe substrate, but to inject electrons to the n-type layer. The deviceof FIG. 8 should have a direct bandgap n-type layer, because therecombinations in that layer are between thermal electrons and thermalholes.

3.2. Implementation using pure Ge and/or Ge_(1-z)C_(z) Random Alloys onSi Substrates

With very heavy n-type doping, the Fermi level may be positioned wellabove the lowest conduction band edge, which makes the lower energystates in the L-valley of the conduction band of germanium to be fullyoccupied. The deeper the Fermi level is inside the conduction band ofgermanium, the smaller the energy necessary to make electrons populatethe bottom of the Γ-valley. With high enough n-type doping, it could bepossible to position the Fermi level above the bottom of the Γ-valley.In that situation direct optoelectronic transitions across the bandgapbecome possible even with thermal electrons, provided that free holesare available to recombine with those electrons.

3.3. Implementation usig (Si_(1-y)C_(y))_(m)—(Gq_(1-z))_(n)Superlattices on Si Substrates

Heavy n-type doping effects can also be exploited in the minibands of(Si_(1-y)C_(y))₅—(Ge_(1-z)C_(z))₅ superlattices, in the same way that itis for pure Ge films. If the lowest interband transition is indirect,but that a direct transition is within a very short distance (inenergy), then that difference can be partially or totally bridged withheavy n-type doping, i.e., the Fermi level can be positioned very closeor even above the Γ-valley of the conduction miniband, and thus theΓ-valley can be populated with thermal electrons. Radiativerecombination leads to the emission of photons with energy correspondingto the difference between the top of the valence band and the Fermilevel inside the conduction band. Such high concentration of impuritiesmay result in increased band mixing and charge carrier scattering, whichcould be advantageous to increase the oscillator strength of directtransitions across the gap of the miniband.

3.4. Implementation using Si₂Sn₂C and/or Ge₃SnC on Si Substrates

As mentioned before, these random alloys have direct bandgaps, andtherefore do not require heavy n-type doping of the active region, noravalanching to bring electrons to the Γ-valley to make efficientradiative recombinations possible. Information about these materials asprovided by P. Zhang, V. H. Crespi, E. Chang, S. G. Louie, M. L. Cohen,“Theory of metastable group-IV alloys from CVD precursors”, Phys. Rev.B, Vol. 64, pp. 235201, 2001; leads to the expectation that they can beincorporated into devices for light emission and light detection,including APDs and ALEDs, and that can be monolithically integrated withsub-100 nm CMOS as disclosed in the present invention. Their bandgapsare direct and small (0.625 eV for Si₂Sn₂C and 0.312 eV for Ge₃SnC)compared to silicon, thus opening new possibilities for detection in theMid-Wavelength Infra-Red (MWIR). It is also possible to combine thesetwo materials into superlattices, and vary the bandgap continuouslybetween 0.312 eV and 0.625 eV. In addition it is also possible to makesuperlattices of Si₂Sn₂C (and/or Ge₃SnC) and Si, SiGe, SiGeC etc. At themoment the band structures of such materials are not known.

3.5. Implementation using Amorphous Si, Ge SiGe Light Emitting Layers

This approach attempts to combine the best possible aspects of thecrystalline silicon for being the acceleration region, in which mobilityis critical, and the avalanching layer, which does not need to have ahigh mobility. In fact it is useful to have a high probability of impactionization, thus scattering, which is typical of non-single-crystalmaterials, and which is helped by heavy doping. As mentioned before, thelayers in which light emission through impact ionization takes place,might not be single crystalline, but rather, a thin film, for exampleless than 20 nm, of a-Si, or a-Ge, or a-SiGe. The amorphous layers,should be heavily doped to maximize the probability of scattering andimpact ionization. Carriers accelerated in the underlying crystallineCMOS layers, will undergo impact ionization the moment they enter theseheavily doped amorphous layers.

Amorphous layers are interesting because their thickness is notrestricted by strain, and because amorphous layers have modified bandstructures with respect to the single crystalline form of the samematerial: for example a-Si has a direct bandgap that is slightly largerthan that of c-Si.

3.6. Devices using Non-Group IV Elements on Silicon Substrates

As mentioned earlier, there a few direct bandgap materials, that havebeen epitaxially grown on silicon, but that are not made of Group-IVcomponents: CuIn_(1-x)Ga_(x)S₂ with bandgap varying linearly with Gacontent from 1.5 eV (x=0) to 2.5 eV (x=1) as demonstrated by H. Metzner,J. Cieslak, J. Eberhardt, Th. Hahn, M. Mjiller, U. Kaiser, A. Chuvilin,U. Reislöhner, and W. Witthuhn, R. Goldhahn and F. Hudert, J. Kräuβlich,“Epitaxial CuIn_(1-x)Ga_(x)S₂ on S(111): A perfectly lattice-matchedsystem for x≈0.5”; Appl. Phys. Lett., Vol. 83, No. 8, 25 Aug. 2003, pp.1563-1565, and SiCAlN with a 3.2 eV bandgap as demonstrated by JohnTolle, R. Roucka, P. A. Crozier, A. V. G Chizmeshya, I. S. T. Tsong, andJ. Kouvetakis, “Growth of SiCAlN on Si(111) via a crystalline oxideinterface”, Appl. Phys. Lett., Vol. 81, No. 12, 16 Sep. 2002, pp.2181-2183.

GaSe, has been experimentally demonstrated by Reiner Rudolph, ChristianPettenkofer, Aaron A. Bostwick, Jonathan A. Adams, Fumio Ohuchi,Maijorie A. Olmstead, Bengt Jaeckel, Andreas Klein, and WolframJaegermann, “Electronic structure of the Si(111):GaSe van der Waals-likesurface termination”, New Journal of Physics7(2005)108; to beepitaxially compatible, and having an interface with (111) silicon, inwhich there are no energy levels in the gap of silicon. The same hasbeen found to be true for AlSe by J. A. Adams, A. Bostwick, T. Ohta,Fumio S. Ohuchi, and Marjorie A. Olmstead, “Heterointerface formation ofaluminum selenide with silicon: Electronic and atomic structure ofSi(111):AlSe”, Phys. Rev. B 71, 195308 2005. Both these materials havebandgaps larger than silicon, thus allowing for the modification of thewavelength of emission.

Iron-Silicide (β-FeSi₂) has been identified as a direct bandgapsemiconductor that can be formed on silicon, potentially compatible withCMOS, and that has a bandgap suitable for light emission and absorptionof the wavelength bands relevant to fiber optics telecommunications.However until now light experimental emission from this material has notmet the high expectations regarding efficiency. The attempts done so farrely on conventional designs for light emitting devices, such as thoseby M. Takauji, C. Li, T. Suemasu, and F. Hasegawa, “Fabrication ofβ-Si/p-FeSi₂/n-Si Double-Heterostructure Light-Emitting Diode byMolecular Beam Epitaxy”, Jpn. J. Appl. Phys., Vol. 44, No. 4B, 2005, pp.2483-2486.

The present invention enables the utilization of this material within adevice design in which the light emission takes place by interbandtransitions induced by impact ionization inside this material. Itinsertion in the CMOS process flow follows the guidelines devised forSiGeC films, including the p-type doping by Boron as identified by Y.Terai, Y. Maeda, “Enhancement of 1.54 μm photoluminescence observed inAl-doped β-FeSi₂”, Appl. Phys. Lett., Vol. 84, No. 6, 9 Feb. 2004, pp.903-905. It can also be used with bulk, thick-film SOI, thin-film SOI,and in conjunction with “energy filtering” layers.

The design superlattices incorporating some these materials,CuIn_(1-x)Ga_(x)S₂, SiCAlN, GaSe, AlSe, β-FeSi₂, Si₂Sn₂C and Ge₃SnC,should allow the bandgap to be varied between 0.312 eV and 3.2 eV. Withthe present invention, it is possible to envision stacks of such layersfor light emission and/or light absorption, each set of layers in thestack operating in a different wavelength or range of wavelengths,monolithically integrated with CMOS.

3.7. Devices using Group IV Elements (Si, Ge, C, Sn) on Ge Substrates

This device and method of fabrication of the present invention is highlysuitable to be applied to Ge substrates, such as bulk Ge and/or GOI,because of the 0.8 eV direct bandgap. One way to take advantage would beto have the avalanching region as heavily n-type doped as possible,because that would make low energy states unavailable to the hotelectrons. The heavy n-type doping will fill up as many states aspossible in the bottom of the conduction band of Ge or in the conductionmini band of a superlattice. Ideally all the states in the indirectband(s) would be completely filled up, so that external electric fieldscan inject electrons directly into the direct band(s). The heavier thedoping and thus thermal electron population, the more likely it is thatany hot electron relaxation is indeed across the 0.8 eV direct bandgap,rather than across the 0.66 eV indirect bandgap.

The smaller the energy difference between the Fermi-level in theindirect band and the bottom of the lowest lying direct band, the lowerthe energy that electrons need to gain in order to move into the directband and thus have a very high probability of participating in radiativetransitions. In this case it is possible that increasing the appliedvoltage, that is, increasing the maximum energy attainable by theelectrons, results in an increase of the energy of the radiativetransitions, thereby resulting in a “blue shift” of the photons emitted.

Additionally, using bulk Germanium or GOI substrates enables the growthepitaxial films containing Sn, and consequently enables radiativeinterband transitions emitting photons in the far infrared.

Due to the low processing temperature of germanium, and especially tothe ease with which the surface of a germanium substrate can be cleanedand made ready for epitaxy, it is then straightforward to have more thanone epitaxial growth, and it becomes possible to have multiple epitaxialdevices, grown on adjacent active areas, with different materials and/ordoping/heterojunction profiles, optimized for different wavelengthranges of the electrmagnetic spectrum.

3.8. Implementation with Pure Ge Device on Ge-Based Substrates

As mentioned above, a pure Ge layer with heavy n-type doping can havethe Fermi level very close or above the Γ-valley. This enables radiativedirect transitions through the recombination of thermal electrons andholes. If the heaviest possible n-type doping not be enough to bring theFermi level close enough to edge of the Γ-valley, then radiative directtransitions between electrons in the Γ-valley and holes at the top ofthe valence band can still take place by populating the F-valley withhot electrons. As also mentioned earlier this is easily achieved with adevice architecture in which impact ionization takes place inside theheavy n-type doped layer.

3.9. Implementation using Group IV Elements on Ge-Based Substrates

Some GeSn alloys have been shown by M. R. Bauer, C. S. Cook, P. Aella,J. Tolle, and J. Kouvetakis, P. A. Crozier, A. V. G. Chizmeshya, and D.J. Smith, S. Zollner, “SnGe superstructure materials for Si-basedinfrared optoelectronics”, Appl. Phys. Lett., Vol. 83, No. 17, 27 Oct.2003, pp. 3489-3491; by H. Pérez Ladrin de Guevara, A. G. Rodriguez, H.Navarro-Contreras, and M. A. Vidal, “Ge_(1-x)S_(x), alloyspseudomorphically grown on Ge(001)”, Appl. Phys. Lett., Vol. 83, No. 24,15 Dec. 2003, pp. 4942-4944; and by G. He and H. A. Atwater, “InterbandTransitions in Sn_(x)Ge_(1-x) Alloys”, Phys. Rev. Lett., Vol. 79, No.10, 8 Sept. 1997, pp. 1937-1940, have direct bandgaps; and some Ge/Snsuperlattices have also been show by D. Munzar, and N. E. Christensen,“Electronic Structure of Sn/Ge superlattices”, Phys. Rev. B, Vol. 49,N0. 16, 15 Apr. 1994-II, pp. 11238-11247, Table V in page 11242, havedirect bandgaps. Because Sn atoms are larger than Ge, it is alsoconceivable that Si and C atoms may be added for partial or total straincompensation, thereby enhancing the critical thickness. The followingare examples of superlattices feasible to grow on germanium substrates:(Si_(1-y)Ge_(y))_(m)—(Ge_(1-z)Sn_(z))_(n);(Si_(1-y)Sn_(y))_(m)—(Ge_(1-z)Sn_(z))_(n);(Si_(1-y)C_(y))_(m)—(Ge_(1-z)Sn_(z))_(n);(Si_(1-y)Ge_(y))_(m)—(Sn_(1-z)C_(z))_(n); and(C_(1-y)Ge_(y))_(m)—(Ge_(1-z)Sn_(z))_(n).

3.10. Devices using Non-Group IV Elements on Germanium Substrates

It has been long known that germanium is a good starting substrate forthe epitaxial growth of GaAs films and devices. Naturally, the currentinvention can also incorporate such direct bandgap films. The films canhave the conventional profiles for light emitting devices, or can alsobe engineered to be thin and highly doped for light emission throughimpact ionization in said films.

4. Application of ALEDs/Lixels

4.1. Solid-State Lighting (SSL)

ALEDs built with direct or pseudo-direct bandgap materials should havevery high power efficiency, approaching that of conventional directbandgap materials used for Solid-State Lighting. For this application,it would not be necessary to have a monolithic integration of the ALEDdevices with CMOS, and therefore the process flow would be much simplerand cheaper.

The advantages over conventional materials and devices for SSL would bemany, starting with production costs several order of magnitude lower,especially when considering that ALEDs can be fabricated on standard 300mm silicon substrates, with all the well established silicon processtechnologies and equipment, while the state of the art devices for SSLare made on 3″ or 4″ expensive substrates, such as sapphire.

4.2. Operation as Light Emitter or Light Absorber

The same device that has been described as a light emitter, when biasedbelow the breakdown voltage can also be operated as an avalanchephoto-detector. The exact same photonic layers (APD/ALED layers) thatare part of the cells described in all Pixel/Lixel designs of aco-pending application can be operated as light sensors or lightemitters. To be operated as an APD for light sensing, the voltageapplied to the terminals of the APD/ALED must be below the breakdownvoltage. With the appropriate circuitry at the periphery of thesensor/emitter matrix, the total current flowing through the APD can becontrolled to suit different conditions of illumination. To be operatedas an ALED, for light emission, the voltage applied to the terminals ofthe APD/ALED must be equal or above the breakdown voltage. With theappropriate circuitry at the periphery of the matrix, the total currentflowing through the ALED can be controlled to prevent damage to thedevice. A matrix of ALEDs can have different applications.

4.3. Application of ALEDs/Lixels for Displays

If the efficiency of light emission and electrical power dissipation areappropriate, a matrix of light emitting elements (Lixels) can be scaledto make displays with an “Active Matrix” (active addressing) of “ActivePixels/Lixels”, that is, a matrix in which each Pixel/Lixel emits light,and in which each pixel has its own turn on/off switch (a MOSFETdevice). Full color displays can be achieved by making a color filtermosaic, such as the Bayer pattern, which is identical to that used forcolor image sensing. The ability of making extremely small activepixels, compared to the usual Pixel/Lixel sizes of conventional flatpanel displays, enables the fabrication of small displays with very highresolution, and thus very high image/video quality. The very small pixelsize could be used to engineer display architectures in which a“Macro-Pixel/Lixel” for a particular primary color, is itself a matrixof many minimum-size Pixels/Lixels. This architecture makes possible todefine the light intensity, and dynamic range, by the number ofminimum-size Pixels/Lixels that are “ON” inside a Macro-Pixel/Lixel. Forexample, a Macro-Pixel/Lixel composed of a matrix of 256×256minimum-size Pixels/Lixels can produce 256×256=65,536 levels of lightintensity, which corresponds to a 16 bit dynamic range. This is a veryconservative estimate because it assumes that each Pixel/Lixel can onlybe either “ON” or “OFF”. If each Pixel/Lixel could have 4 differentlevels of light intensity (corresponding to 4 different appliedvoltages), then the dynamic range would be 18 bit. At the moment, thebest dynamic range for the most common flat panel display technologies,Plasma Displays and Liquid Crystal Displays are in the 10 to 12 bitrange.

4.4. Application of Pixels/Lixels for a Dual-Mode Sensor/Emitter Matrix

A matrix of Pixels/Lixels, for sensing/emitting light can be used in acamera for video or still photography, with the ability to alternate(through software control) image acquisition and light emission. Thisalternate functionality can be used for the following applications:3D-Imaging

The measurement of the “Time-Of-Flight” (TOF) between the emission ofphotons and the detection of the reflected photons can be used for3-dimensional imaging. This includes the possibility to measure multiplereflections, and thus see “behind” the objects in the filed of view. Themeasurement of the time of flight can be made with lightemission/detection using only some or all of the primary colors. If onlythe IR is used, the image as seen by the human eye and recorded bytypical cameras (film and digital), will not be disturbed. The fact thatthe light used for this purpose reaches the objects in the field of viewthrough the lens of the camera (traveling in the opposite direction ofthe light that forms an image on the sensor) may have benefits in termsof power needed as well as better control of secondary reflections. Thiscan be especially useful when using tele-photo lens to capture 3D imagesof very far away objects.

ALEDs/Lixels as a Fully Integrated Flash

In this case the light of the flash travels through the lenses of thecamera, and illuminates directly only what is in the field of view ofthe lenses, regardless of their type (wide angle, macro, telephoto,etc.). In turn this should decrease the optical power requirements,compared to a conventional flash, which emits light isotropically. It ispossible to control the color(s) and color temperature of the flash, bycontrolling the light intensity that goes through each type of primarycolor filter (R, G, B, IR). This is radically different fromconventional flashes for cameras.

Monolithically Integrated Optoelectronic Transceiver

With appropriate the material/composition for the epitaxially depositedlight emitting film, it is possible to have light absorption & emissionin the infrared, namely in the 1.3 μm to 1.55 μm range, which are thewavelengths used in fiber optics communications. Therefore, it becomespossible to fabricate CMOS integrated circuits that can directly receivelight signals from, and emit light signals to, optical fibers, therebyenabling low cost technology alternative to solutions based on III/Vcompound semiconductors.

In the present disclosure, several materials have been mentioned as goodcandidates to cover these wavelength ranges:(Ge_(1-z)C_(z))—(Si_(1-y)C_(y))₅, β-FeSi₂, Si₂Sn₂C, and Ge₃SnC.

1. A photonic device monolithically integrated with CMOS, includingsub-100 nm CMOS, with active layers comprising acceleration regions,light emission and absorption layers, and optional energy filteringregions, wherein light emission or absorption is controlled by anapplied voltage to deposited films on a pre-defined CMOS active area ofa substrate, such as bulk Si, bulk Ge, Thick-Film SOI, Thin-Film SOI,Thin-Film GOI.
 2. A photonic device according to claim 1, made onsilicon substrates, that can have mono-layer flatness the photonicactive layer film is made of a pseudomorphic indirect bandgap material,such as a random alloys of Si, Ge, SiGe, SiC, SiSn, GeC, GeSn, GeCSn,SiGeC, SiGeCSn, etc, and in which light is emitted by impact ionizationof hot carriers inside the deposited film. the photonic active layerfilm is made of a pseudomorphic pseudo-direct bandgap material, such asa superlattices of Si, Ge, SiGe, SiC, SiSn, GeC, GeSn, GeCSn, SiGeC,SiGeCSn, etc, and in which light is emitted by forward biasing of thefilm or by impact ionization of hot carriers inside the deposited film.the photonic active layer film is made of a pseudomorphic direct bandgapmaterial, such as Si₂Sn₂C and/or Ge₃SnC, and in which light is emittedby forward biasing of the film or by impact ionization of hot carriersinside the deposited film. the photonic active layer film is made of apseudomorphic direct bandgap material not belonging to the Group IV,such as CuIn_(1-x)Ga_(x)S₂, or SiCAlN, and in which light is emitted byforward biasing of the film or by impact ionization of hot carriersinside the deposited film. photonic active layers can also be operatedas light sensors, including in the avalanche mode. the lower theabsolute bandgap of the material in which avalanche takes place, thelower the required applied voltage to initiate an avalanche. newminority carrier injection mechanism for efficient radiativerecombination in direct, pseudo-direct, and indirect bandgap activeregions. possibility of stacking epitaxial direct and pseudo-directbandgap materials, with bandgaps from 0.312 eV to 3.2 eV, formingquantum wells and/or superlattices.
 3. A photonic device according toclaim 1, made on germanium substrates, that can have mono-layer flatnesswherein: the photonic active layer film is made of a pseudomorphicindirect bandgap material, such as a random alloys of Si, Ge, SiGe, SiC,SiSn, GeC, GeSn, GeCSn, SiGeC, SiGeCSn, etc, and in which light isemitted by impact ionization of hot carriers inside the deposited film.the photonic active layer film is made of a pseudomorphic pseudo-directbandgap material, such as a superlattices of Si, Ge, SiGe, SiC, SiSn,GeC, GeSn, GeCSn, SiGeC, SiGeCSn, etc, and in which light is emitted byforward biasing of the film or by impact ionization of hot carriersinside the deposited film. the photonic active layer film is made of apseudomorphic direct bandgap material, such as GeSn random alloys and/orsuperlattices, and in which light is emitted by forward biasing of thefilm or by impact ionization of hot carriers inside the deposited film.the photonic active layer film is made of a pseudomorphic direct bandgapmaterial not belonging to the Group IV, such as AlGaAs alloys andsuperlattices, and in which light is emitted by forward biasing of thefilm or by impact ionization of hot carriers inside the deposited film.phothotonic active layers can also be operated as light sensors,including in the avalanche mode. the lower the absolute bandgap of thematerial in which avalanche takes place, the lower the required appliedvoltage to initiate an avalanche. new minority carrier injectionmechanism for efficient radiative recombination in direct,pseudo-direct, and indirect bandgap active regions. possibility ofstacking epitaxial direct and pseudo-direct bandgap materials, withbandgaps from 0.312 eV to 3.2 eV, forming quantum wells and/orsuperlattices.
 4. A photonic device according to claim 1 in which lightis emitted from the epitaxial film, perpendicularly to its surface andthe substrate.
 5. A photonic device according to claim 1, wherein theapplied bias is below the breakdown voltage can be operated as anavalanche photo-diode.
 6. A photonic device according to claim 1,fabricated on Thin-Film SOI or Thin-Film GOI, embedded in an opticalvertical cavity, wherein: light emission and/or absorption fromback-side only; and light emission and/or absorption from front-side andback-side
 7. Layout cell, suitable for replication to form largematrices, incorporating one MOSFET and the device of claim 1 wherein:identical doping type of source/drain regions of said MOSFET and bottomelectrode of the photonic active layer. the contact to the bottomelectrode of the photonic active layer is made by electricallyconnecting the active area under the photonic layer with thesource/drain region of the adjacent MOSFET. in bulk or thick-film SOI orGOI substrates, connecting the active area under the photonic layer withthe source/drain region of the adjacent MOSFET, is accomplished with a“well implant” with the same doping type of the active area and of thesource/drain regions of said MOSFET. in bulk or thick-film SOI or GOIsubstrates the active area on which the photonic layer is deposited issurrounded by isolation (for example shallow trenches), and the “wellimplant” provides a conductive path under the oxide-filed trench in aregion that is immediately adjacent to the source/drain region of saidMOSFET; and contact to top electrode of the photonic active layer can becommon to all cells in a matrix.